Non-volatile sampler

ABSTRACT

A non-volatile sampler including a row line for receiving an input signal to be sampled, the row line intersecting a number of column lines, non-volatile storage elements being disposed at intersections between the row line and the column lines; a bias voltage source connected to the column lines, the bias voltage source for selectively applying a bias voltage to at least one of the non-volatile storage elements to cause the at least one of the storage elements to store a sample of the input signal at the instance the bias voltage is applied.

BACKGROUND

Physical computing systems and other electronic devices often make useof both volatile and non-volatile types of memory. Volatile memory losesits data when power is no longer being supplied. Non-volatile memory canstill retain data even when the power supply has been deactivated ordisconnected. However, volatile types of memory are able to operate atmuch faster rates. Particularly, data can be written to a volatilememory device much faster than it can be written to a non-volatilememory device.

One type of electronic device which makes use of both volatile andnon-volatile memory is a sampling system. A sampling system receives asignal from a sensor in an analog format. An analog signal is a timevarying signal that can assume any continuous value. This is opposed toa digital signal which can only assume a discrete set of values. Atypical sampling system uses an Analog-to-Digital Converter (ADC) toplace the received analog signal into a digital form. This digital datacan then be stored onto a non-volatile memory device. However, manytypes of sampling systems sample data at a rate higher than can bewritten to typical non-volatile memory devices. Consequently, samplingsystems typically require use of a high speed volatile type of memory toact as a buffer. The use of both the ADC and the high speed volatilememory add to the cost and size requirements of the sampling system.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various embodiments of theprinciples described herein and are a part of the specification. Theillustrated embodiments are merely examples and do not limit the scopeof the claims.

FIGS. 1A and 1B are diagrams showing illustrative components of asampling system, according to one example of principles describedherein.

FIG. 2 is a diagram of an illustrative crossbar array, according to oneexample of principles described herein.

FIG. 3A is a diagram showing an illustrative high speed non-volatilememory device, according to one example of principles described herein.

FIG. 3B is a diagram showing an illustrative graph of clock signaltiming for the memory device of FIG. 3A, according to one example ofprinciples described herein.

FIG. 4 is a diagram showing an illustrative graph of a relationshipbetween an input signal and bias voltages, according to one example ofprinciples described herein.

FIG. 5 is a diagram showing an illustrative high speed non-volatilememory device able to store digitized values, according to one exampleof principles described herein.

FIG. 6 is a diagram showing an illustrative graph of a relationshipbetween an input signal and different bias voltages, according to oneexample of principles described herein.

FIGS. 7A and 7B are diagrams showing an illustrative sampling system,according to one example of principles described herein.

FIG. 8 is a flowchart of a method for storing data on a high speednon-volatile memory device, according to one example of principlesdescribed herein.

Throughout the drawings, identical reference numbers designate similar,but not necessarily identical, elements.

DETAILED DESCRIPTION

As mentioned above, one type of electronic device which makes use ofboth volatile and non-volatile memory is a sampling system. A samplingsystem receives a signal from a sensor in an analog form. An analogsignal is a time varying signal that can assume any continuous value.This is opposed to a digital signal which can only assume a discrete setof values, usually ones and zeros that can be organized into digital“words” that represent any desired value.

A typical sampling system uses an Analog-to-Digital Converter (ADC) toplace the received analog signal into a digital form. This digital datacan then be stored in a non-volatile memory device. However, many typesof sampling systems sample data at a higher rate than can be written totypical non-volatile memory devices. Consequently, sampling systemstypically require use of a high speed volatile type of memory to act asa buffer. The use of both the ADC and the high speed volatile memorybuffer add to the cost and size requirements of the sampling system.

One type of non-volatile memory structure is a crossbar memory array. Acrossbar memory array includes a first set of parallel linesintersecting a second set of parallel wire lines. For purposes ofillustration, one of these sets of parallel lines will be referred to asrow lines, and the other set of lines will be referred to as columnlines. However, the term row line or column line does not imply anyspecific orientation.

In a crossbar memory array, storage elements are placed at intersectionsbetween row lines and column lines. One type of storage element whichcan be used is a memristive device. A memristive device will maintainits resistive value until a programming condition is applied. Aprogramming condition can be a voltage above a threshold voltage.Voltages under the threshold voltage will have a negligible effect onthe resistance of a memristive memory element. However, voltages abovethe threshold voltage will cause a change in the resistance value of thememristive device. The degree of change is dependent upon the voltagevalue and the duration of time which that voltage is applied.

Storage elements such as memristive devices can be used to store data ina digital format. For example, a high resistive state can represent adigital ‘0’ and a low resistive state can represent a digital ‘1’.Alternatively, memristive devices can be used to store an analogrepresentation of an analog signal. Memristive devices may assume anyresistive value between the high resistive state and the low resistivestate. Thus, the resistive value of a memristive device can be used torepresent the instantaneous value of a sampled analog signal over arange of values.

The present specification relates to a method and system for using acrossbar memory array to sample an analog signal. According to certainillustrative examples, a number of non-volatile storage elements areplaced along a row line. Each non-volatile storage element is alsoconnected to one of a set of column lines running perpendicular to therow line. Although various types of non-volatile storage elements can beused in a sampler embodying principles described herein, thisspecification will give a description using the example of a memristivedevice.

As the row line receives an input signal, bias voltages can beselectively applied to the column lines. The bias voltage can be suchthat the voltage drop across the memristive device is above thethreshold voltage for changing the state of a memristive device.Additionally, the voltage drop across the memristive device will bedependent on the value of the input signal at the instant which the biasvoltage is applied to one of the column lines. Thus, a sample of theinput signal at the time in which a bias voltage is applied to a columnline connected to a memristive device is stored in that memristivedevice.

Through use of a sampling system embodying principles described herein.Sampled data can be stored directly to a non-volatile storage elementwithout the need for an ADC or an additional memory to be used as abuffer. Certain non-volatile storage elements such as memristive devicesare able to switch at high speeds, thus a sampling system using suchhigh speed devices can be used to store data which has been sampled athigh resolution. Additionally, memristive devices are relatively small.Thus, they can store a high volume of data while using little space.

In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present systems and methods. It will be apparent,however, to one skilled in the art that the present apparatus, systemsand methods may be practiced without these specific details. Referencein the specification to “an embodiment,” “an example” or similarlanguage means that a particular feature, structure, or characteristicdescribed in connection with the embodiment or example is included in atleast that one embodiment, but not necessarily in other embodiments. Thevarious instances of the phrase “in one embodiment” or similar phrasesin various places in the specification are not necessarily all referringto the same embodiment.

Throughout this specification and in the appended claims, the term“non-volatile storage element” is to be broadly interpreted as a devicewhich is able to switch its resistance state to store data. This data isretained with or without power. A non-volatile storage element, asdefined here, can store data in both analog and digital form. Oneexample of a non-volatile storage element is a memristive device.Additional examples include devices based on phase change materials andmagnetic-based devices such as spin-transfer torque memory.

Referring now to the figures, FIGS. 1A and 1B are diagrams showingillustrative components of sampling systems (100, 110). FIG. 1Aillustrates a typical sampling system (100). As mentioned above, atypical sampling system (100) includes an ADC (104), a buffer (106) anda non-volatile memory device (108).

The ADC (104) receives an analog signal from a signal source (102) suchas a sensor and converts that analog signal into a digital signal. Thesensor may be, for example, a light sensor on a camera, or an acousticwave sensor such as a microphone. The ADC (104) then sends the digitizedsignal to a buffer (106).

The buffer (106) is a high speed memory device that is able to bewritten to at high rates. This allows the analog signal from the sensorto be sampled at high rates. However, to be able to write at highspeeds, the buffer (106) is typically made of a volatile type of memory.Thus, the data stored in the buffer (106) is transferred to anon-volatile memory storage device (108) so that the data can beretained even when power is no longer being supplied.

FIG. 1B is a diagram of a sampling system (110) embodying principlesdescribed herein. As mentioned above, a crossbar memory array may act asa non-volatile sampler (112). This eliminates the need for a separateADC (104) circuit and a buffer (106).

FIG. 2 is a diagram showing an illustrative crossbar memory array (200).According to certain illustrative examples, the crossbar memory array(200) may include an upper set of upper lines (202) which may generallybe in parallel. Additionally, a lower set of lines (204) is generallyperpendicular to, and intersects, the upper lines (202). Programmablecrosspoint or memory devices (206) are formed at the intersectionsbetween an upper line (208) and a lower line (210).

According to certain illustrative examples, the programmable crosspointdevices (206) may be memristive devices or other non-volatile storageelements as defined above. Memristive devices exhibit a “memory” of pastelectrical conditions. For example, a memristive device may include amatrix material that contains mobile dopants. These dopants can be movedwithin a matrix to dynamically alter the electrical operation of anelectrical device, such as the resistance of that device.

The motion of dopants can be induced by the application of a programmingcondition such as an applied electrical voltage across a suitablematrix. The programming voltage generates a relatively high electricalfield through the memristive matrix and alters the distribution ofdopants. After removal of the electrical field, the location andcharacteristics of the dopants remain stable until the application ofanother programming electrical field.

As indicated, by changing the dopant configurations within a memristivematrix, the electrical resistance of the device may be altered. Thememristive device is then read by applying a lower reading voltage whichallows the internal electrical resistance of the memristive device to besensed but does not generate a high enough electrical field to causesignificant dopant motion. Consequently, the state of the memristivedevice may remain stable over long time periods and through multipleread cycles.

According to certain illustrative examples, the crossbar memory array(200) may be used to form a non-volatile memory array. As noted above,non-volatile memory has the characteristic of not losing its contentswhen no power is being supplied. Each of the programmable crosspointdevices (206) is used to represent one or more bits of data. Althoughindividual crossbar lines (208, 210) in FIG. 2 are shown withrectangular cross sections, crossbars may also have square, circular,elliptical, or more complex cross sections. The lines may also have manydifferent widths, diameters, aspect ratios and/or eccentricities. Thecrossbars may be nanowires, sub-microscale wires, microscale wires, orwires with larger dimensions.

According to certain illustrative examples, the crossbar memory array(200) may be integrated into a Complimentary Metal-Oxide-Semiconductor(CMOS) circuit or other conventional computer circuitry. Each individualwire segment may be connected to the CMOS circuitry by a via (212). Thevia (212) may be embodied as an electrically conductive path through thevarious substrate materials used in manufacturing the crossbararchitecture. This CMOS circuitry can provide additional functionalityto the memristive device such as input/output functions, buffering,logic, configuration, or other functionality. Multiple crossbar arrayscan be formed over the CMOS circuitry to create a multilayer circuit.

FIG. 3A is a diagram showing an illustrative high speed non-volatilesampler (300). According to certain illustrative examples, a row line(310) receives an input signal (302) from a signal source. The inputsignal (302) can be in the form of a voltage. The input signal (302) canalso be scaled so that at its highest value, it is still not greaterthan the threshold voltage required to change the resistive state of thememristors (304) along the row line (310). The following describes themanner in which a representation of the input signal (302) is sampledand stored in the memristors (304).

During normal operation, the time-varying input signal (302) is appliedto one terminal of each of the memristors (304) along the row line(310). However, because the amplitude of the signal is smaller than thethreshold voltages required to change the resistive state of thememristors, the memristors remain unaffected. Additionally, all switches(306) connecting the bias voltage (308) to the memristors (304) are inan open position.

At a first sampling instant, switch 1 (306-1) is closed. When switch 1is closed, the bias voltage (308) is applied to a terminal of memristor1 (304-1). The voltage across memristor 1 (304-1) at this samplinginstant will be the difference in voltage between the bias voltage (308)and the voltage value of the input signal (302) at this samplinginstant. The bias voltage (308) is of a value such that it is above thethreshold voltage required to change the state of the memristors (304).Thus, the difference between the bias voltage (308) and any possiblevalue of the input signal (302) will cause memristor 1 (304-1) to changeits resistive value. The newly assumed resistive value of memristor 1(304-1) is dependent upon the signal value of the input signal (302) atthe instant which the bias voltage was applied to the column line (312)connected to memristor 1 (304-1). Thus, the newly assumed resistivevalue of memristor 1 (304-1) is representative of the input signal (302)value at this sampling instant.

During subsequent sampling instants, switch 2 (306-2) is opened to storea value of the input signal in memristor 2 (304-2), switch 3 (306-3) isopened to store a value of the input signal in memristor 3 (304-3), andswitch 4 (306-4) is opened to store a value of the input signal inmemristor 4 (304-4).

Although FIG. 3A only illustrates four memristors along the row line(310), a practical non-volatile sampler (300) may have a greater numberof memristors along a row line (310). Additionally, a non-volatilesampler (300) can have multiple row lines (310) intersecting the columnlines (312). The input signal (302) can then be switched between themultiple row lines (310).

FIG. 3B is a diagram showing an illustrative graph (314) of clock signaltiming for the memory device of FIG. 3A. The graph (314) illustrates thetiming signals for switch 1 (306-1), switch 2 (306-2), switch 3 (306-3),and switch 4 (306-4). The horizontal axis represents time. The clocksignal controls when a switch is open or closed. In this example, whenthe clock signal is high, the switch is open. Conversely, when the clocksignal is low, the switch is closed.

At sampling instant 1 (316-1), the clock signal for switch 1 (306-1)goes high for a short duration of time. This places switch 1 (306-1) inan open position for a short duration of time. During this time, thebias voltage is applied to memristor 1 (304-1), causing memristor 1(304-1) to store the value of the input signal at sampling instant 1(316-1). This same process occurs at sampling instant 2 (316-2),sampling instant 3 (316-3) and sampling instant 4 (316-4).

FIG. 4 is a diagram showing an illustrative graph (400) of arelationship between an input signal (402) and a bias voltage (408). Thevertical axis of the graph (400) represents voltage (402) and thehorizontal axis of the graph (400) represents time (404). The inputsignal (406) is shown as having a varying voltage value over time.

According to certain illustrative examples, the bias voltage (408) is apositive voltage. The positive bias voltage (408) is such that thesmallest difference between the input signal (406) and the bias voltageis larger than the threshold voltage of the memristors. At one samplinginstant, when the input signal (406) is near its peak, the voltagedifference (410-1) is relatively small. However, it will still be abovethe threshold voltage and will therefore change the state of thememristor. In a different sampling instant, when the input signal (406)is near its bottom, the voltage difference (410-2) is relatively large.In some examples, the bias voltages (408) may be negative voltages. Thevoltage across the memristor will still be the difference between theinput signal (406) and the negative bias voltage at the instant whichthe negative bias voltage is applied to the memristor.

Storing a sample of an analog value in an analog manner allows data tobe sampled at high rates. This is because an ADC is not used to convertthe analog signal into a digital signal. However, in some cases, it canbe beneficial to use digitized representations of signals. Consequently,in some examples, the non-volatile sampler can act as an ADC and storesamples of the received input signal as a digitized representation ofthe input signal.

FIG. 5 is a diagram showing an illustrative high speed non-volatilesampler (500) able to store digitized values. According to certainillustrative examples, a set of memristors along a particular row line(510) of a memristive crossbar array can be used to store a digitalvalue (506) of a sample of an input signal (502). Each row line (510) inthe array can be used to store a sample of the input signal (502) at adifferent instant. In the example of FIG. 5, four row lines (510) areused to store four 4-bit samples of the input signal. The following willdescribe the basic operation of such a non-volatile sampler.

The input signal (502) is received into a multiplexer (504). Amultiplexer (504) is a digital circuit designed to receive one input andswitch that input to one of several outputs based on a control signal.The control signal can be designed so that it causes the multiplexer(504) to switch the input signal to each of the outputs in succeedingtime intervals. This procedure is often referred to as timemultiplexing.

At each sampling instant (512), when the input signal is being appliedto a particular row line (510), a different bias voltage (508) isapplied to each of the column lines (516). The different bias voltages(508) can be designed so that based on the value of the sampled inputsignal, the different memristive devices will be either change or beleft unaltered. For example, if the input signal (502) at a particularinstant when combined with a bias voltage (508) is larger than thethreshold voltage, then the memristive device will change. The set ofmemristors along a given row line can then be a digital representationof the sampled input signal.

FIG. 6 is a diagram showing an illustrative graph of a relationshipbetween an input signal and different bias voltages. There are many waysin which the bias voltages of each of the column lines may be set tospecific levels in order to digitally encode a sample of the inputsignal. One way is to use thermometer coding.

Thermometer coding is such that the value represented by a number ofbits is equal to the number of bits in the ON state. For example, a4-bit thermometer coded value can assume 5 discrete values; 0, 1, 2, 3and 4. If none of the bits are in an ON state, then the valuerepresented is zero. If all four of the bits are in an ON state, thenthe value represented is four. The memristive devices along a row linecan be used to represent an ON or OFF state based on their resistivevalues. For example, a high resistive value can represent an OFF stateand a low resistive value can represent an ON state.

When sampling a signal with a non-volatile sampler (e.g. 500, FIG. 5),each of the memristors may be initially set to a high resistive state.If the applied bias voltage applied to a particular memristive devicealong the row line is large enough to change the resistive value of thatmemristive device, then that memristive device is set to a low resistivestate.

In one example, a given input signal value (606) is applied to a rowline having four memristors attached. Each of the memristors is attachedto a different column line having a different bias voltage (608) beingapplied. As mentioned above, the bias voltages (608) can be negative.The difference (610-1) between the positive signal value (606) and thefirst bias voltage (608-1) applied to a first memristor is not enough tochange the value of the first memristor. Thus, the first memristorremains in a high resistive state, representing a digital ‘0’.Similarly, a second memristor has a slightly higher bias voltage (608-2)applied. The difference (610-2) between this bias voltage (608-2) andthe input signal value (606) may still not be enough to change the stateof the second memristor. However, the third memristor has a bias voltage(608-2) applied which is large enough to cause the difference (610-3)between this bias voltage (608-3) and the input signal value (606) toexceed the threshold voltage and change the state of the third memristorto a low resistive state. Thus, the third memristor may represent adigital ‘1’. Similarly, the fourth memristor having an even bigger biasvoltage (608-4) applied will have a difference (610-4) significantenough to change the state of the fourth memristor to represent adigital ‘1’. The final digitized value (612) for this example would thenbe “0011”.

The manner described above for storing a digitized signal in anon-volatile sampler is merely one way of doing so. As will beappreciated by those skilled in the art, other ways of encoding thedigitized value by using different bias voltages applied to differentcolumn lines can be used.

FIGS. 7A and 7B are diagrams showing an illustrative sampling system(700). According to certain illustrative examples, a high speednon-volatile sampler embodying principles described herein is used inconjunction with a pixel sensor for a camera. The camera may be either aphoto camera or a video camera. FIG. 7A shows the relation between thepixel sensor array (702) and the high speed non-volatile sampler (704).The pixel sensor array (702) senses light and sends an analogrepresentation of the sensed light to the high speed non-volatilesampler (704).

FIG. 7B is a diagram illustrating a high speed non-volatile sampler(704) in which each row line is connected to one pixel (706) of thepixel sensor array (702). Thus each row can store, in a non-volatilemanner, the signal received from a particular pixel (706) of the pixelsensor array (702). If storing an analog representation of the receivedsignal, the memristive devices (710) along a particular row line canstore different samples of the input signal which were received atdifferent sampling instants. As mentioned above, this is done byapplying bias voltages to the column lines at different instances intime.

FIG. 8 is a flowchart of a method for storing data on a high speednon-volatile memory device. According to certain illustrative examples,the method includes receiving (block 802) an input signal on a row lineintersecting a set of column lines perpendicular to the row line,non-volatile storage elements being disposed at intersections betweenthe row line and the set of column lines; selectively applying (block804) bias voltages to the set of column lines; and storing (block 806) asample of the input signal in one of the storage elements disposed at anintersection between the row line and one of the set of column lineshaving a bias voltage applied.

In conclusion, through use of a sampling system embodying principlesdescribed herein. Sampled data can be stored directly to a non-volatilestorage element without the need for an ADC or an additional memory tobe used as a buffer. Certain non-volatile storage elements such asmemristive devices are able to switch at high speeds, thus a samplingsystem using such high speed devices can be used to store data which hasbeen sampled at high resolution. Additionally, memristive devices arerelatively small. Thus, they can store a high volume of data while usinglittle space.

The preceding description has been presented only to illustrate anddescribe embodiments and examples of the principles described. Thisdescription is not intended to be exhaustive or to limit theseprinciples to any precise form disclosed. Many modifications andvariations are possible in light of the above teaching.

1. A non-volatile sampler comprising: a row line for receiving an inputsignal to be sampled, said row line intersecting a number of columnlines, non-volatile storage elements being disposed at intersectionsbetween said row line and said column lines; a bias voltage sourceconnected to said set of column lines, said bias voltage source forselectively applying a bias voltage to at least one of said non-volatilestorage elements to cause said at least one of said storage elements tostore a sample of said input signal at the instance said bias voltage isapplied.
 2. The sampler of claim 1, in which said sample of said inputsignal stored in said at least one of said non-volatile storage elementsis an analog representation of said sample of said input signal.
 3. Thesampler of claim 1, in which application of said bias voltage to one ofsaid column lines is time-multiplexed, causing non-volatile storageelements along said row line to store samples of said input signal takenat different times.
 4. The sampler of claim 1, further comprising anumber of additional row lines intersecting said column lines.
 5. Thesampler of claim 4, further comprising a multiplexer to switch saidinput signal between said row line and said number of additional rowlines.
 6. The sampler of claim 1, in which additional non-volatilestorage elements along said row line are used to store a digitizedrepresentation of said sample of said input signal.
 7. The sampler ofclaim 6, in which different bias voltages are applied to differentcolumn lines from said column lines, each of said bias voltagesdetermining a different bit of said digitized representation to bestored in one of said non-volatile storage elements having that biasvoltage applied.
 8. The sampler of claim 1, in which said input signalis received from a pixel sensor.
 9. The sampler of claim 1, in whichsaid non-volatile storage elements are memristive devices.
 10. A methodfor fabricating a non-volatile memory device, the method comprising:disposing non-volatile storage elements at intersections between a rowline and a set of column lines, said row line for receiving an inputsignal on a row line intersecting a set of column lines perpendicular tosaid row line, non-volatile storage elements being disposed atintersections between said row line and said column lines; connectingbias voltages to said set of column lines, said bias voltages forselectively applying a bias voltage to at least one of said non-volatilestorage elements to cause said at least one of said storage elements tostore a sample of said input signal at the instance said bias voltage isapplied.
 11. The method of claim 10, in which said sample of said inputsignal is stored in said one of said non-volatile storage element as ananalog representation of said sample of said input signal.
 12. Themethod of claim 10, in which selectively applying said bias voltage istime-multiplexed, causing non-volatile storage elements along said rowline to store samples of said input signal at different instances. 13.The method of claim 10, further comprising receiving said input signalon a number of additional row lines intersecting said column lines. 14.The method of claim 13, further comprising, with a multiplexer,switching said input signal between said row line and said number ofadditional row lines.
 15. The method of claim 10, further comprising,using additional non-volatile storage elements along said row line areused to store a digitized representation of said sample of said inputsignal.
 16. The method of claim 15, in which different bias voltages areapplied to different column lines from said column lines, each of saidbias voltages determining a different bit of said digitizedrepresentation to be stored in one of said non-volatile storage elementshaving that bias voltage applied.
 17. The method of claim 10, in whichsaid input signal is received from a pixel sensor.
 18. The method ofclaim 10, in which said non-volatile storage elements are memristivedevices.
 19. A non-volatile sampling system comprising: a sensor whichoutputs an analog signal; a row line to receive said analog signal, saidrow line intersecting a set of column lines perpendicular to said rowline, non-volatile storage elements being disposed at intersectionsbetween said row line and said column lines; a bias voltage sourceconnected to said set of column lines, said bias voltage source forselectively applying a bias voltage to at least one of said non-volatilestorage elements to cause said at least one of said storage elements tostore a sample of said analog signal at the instance said bias voltageis applied.
 20. The system of claim 18, in which said analog signal isstored in said one of said non-volatile storage elements as an analogvalue representing said sample of said analog signal.